Hard decision decoding takes a stream of bits from a “threshold detector” stage of a receiver, where each bit is considered definitely “1” or “0.” For example, for binary signaling, received pulses are sampled and the resulting voltages are compared with a single threshold. If a voltage is greater than the threshold it is considered to be definitely a “1” regardless of how close it is to the threshold. If it is less than the threshold value, it is considered to be definitely a “0.”
Soft decision decoding requires a stream of “soft” bits including the “1” or “0” hard decision as well as “confidence” bits providing an indication of the certainty that decision is correct.
An exemplary mechanism for implementing soft decision decoding would be to configure a threshold detector to generate as follows: 000 (definitely 0), 001 (probably 0), 010 (maybe 0), 011 (guess 0), 100 (guess 1), 101 (maybe 1), 110 (probably 1), 111(definitely 1). Where the first bit is the “hard” decision bit and the two bits may be considered to a “confidence” bits with regard to the certainty that the “hard” decision bit has been correctly detected.